On the ethnicity section of the app, I put Hispanic origin since I'm Mexican. However, I'm not sure what to put in the race section since Mexican (or anything of the like) isn't an option. Would I just leave it blank?
Hispanic = ethnicity.
Mexican = nationality (though I assume you're actually American with Mexican ancestry).
Race = your phenotypic traits (i.e. the color of your skin). Mexicans/Hispanics can be Black, White, Metizo (a mixture), or something else. Most hispanic people are white or mixed.
My ethnicity is Hispanic. For race I check the "white/caucasian" box because that what I mostly look like - though most "white" people will never be able to obtain/maintain my perma-"tan." Lol.